How do you build leg strength for climbing?
Squats are great for building general leg strength, while also working many other upper body muscles.
Another great exercise is sprinting. Sprinting can really push you to your limits and give you that leg power you are looking for.
Pistol squats is another great exercise that is very popular among climbers. It's very similar to regular squats, but also quite different, as you will need to put extra effort in balancing yourself on one leg in order to do the squat. To make it easier, you can do it while standing on the edge of a box, and performing the squat there. Part of the difficulty in the pistol squat is doing the squat with your other leg extended out front. So, by minimizing the amount that leg must be out front will make the squat easier to perform, and this is done by standing on something tall.
Calf raises are a great exercise and a personal favorite for building and strengthening the calf muscles. It is also one of the easiest exercises that can be performed. All you need is a step, this can be a staircase step, a weight that you stand on, or anything else with an edge that you can stand on and have your heel touch the ground. Just make sure that what ever you are standing is tall enough so that your calf muscle will actually be extended when your heel touches the ground. To do the exercise, stand on the edge with your toes, lower your heel to the ground, and lift yourself up slowly, hold at the top for a second, and then lower yourself slowly, and repeat.
The bottom line is that building leg strength for climbing is just like building leg strength in general, really. In climbing, you use your legs for certain hooks, jumps, dynamic movements, and stabilization, all of which require general leg strength in most or all of your main leg muscles. The exercises above will work those muscles.
It is also important to keep in mind that climbing itself activates certain muscles that may not be activated in other exercises, and they may be used in certain ways and conditions that are hard to replicate outside of climbing. These situations are also important for training and strengthening your muscles, and not just your leg muscles, so keep on climbing, and add the additional leg exercises as a supplement to your climbing.